Soluble sulfides (H.sub.2 S, HS.sup.-, S.sup.-2) are often found to contaminate water co-produced with petroleum, anaerobic digester effluents and various industrial wastewaters. The source of these sulfides is generally the reduction of sulfates by sulfate reducing bacteria. These bacteria are strict anaerobes which utilize a rather limited number of organic compounds as a source of carbon and energy such as pyruvate, lactate, acetate and ethanol. However, these compounds are end products of the metabolism of fermentive heterotrophs and are readily available in a consortium of bacteria in an anaerobic environment. Therefore, sulfate reducing bacteria are ubiquitous to virtually any anaerobic environment conducive to microbial growth.
The toxicity and corrosive properties of sulfides dictate stringent control of their release into the environment and contact with iron and steel as in tank, pipelines, valves and pumps. The control of sulfide contamination may be approached in two ways. First, sulfide production may be reduced by inhibiting the growth of sulfate reducing bacteria. For example, in the secondary production of petroleum, water used in flooding operations is treated with a biocide to control sulfate reducing bacteria growth in the injection well, reservoir and piping. Since sulfate reducing bacteria are strict anaerobes, aeration of flooding water can also serve to inhibit sulfide production. These measures are of limited effectiveness, however, because sulfate reducing bacteria are sessile bacteria. That is to say, they are generally found attached to a solid surface entrapped with other bacteria in polysaccharide gels produced by "slime-forming" bacteria. Within these gels the sulfate reducing bacteria find themselves in a somewhat protected environment which biocides and oxygen do not effectively penetrate. Of course, biocide treatment is inappropriate in a situation in which the growth of other microorganisms is to be encouraged, such as in an anaerobic digester. In an anaerobic digester, the growth of sulfate reducing bacteria can sometimes be inhibited by fostering competition between the sulfate reducing bacteria and other heterotrophs for the carbon and energy sources favored by the sulfate reducing bacteria. The success of this approach is, however, dependent upon the type of waste being treated.
If sulfide production cannot be prevented, sour water may be treated by a number of physiochemical methods. One of the more common methods is to strip sulfide-laden waters under acidic conditions with steam, flue gas or methane in a packed or plate-type column. In the case of steam or flue gases, the overhead vapors are condensed and the noncondensables (including H.sub.2 S) are incinerated. In the case of methane, the noncondensables are typically sent to an amine system and the methane recycled. Hydrogen sulfide recovered from the methane stripping gas is generally incinerated. Each of these processes converts a water pollution problem into an air pollution problem in that the combustion of H.sub.2 S produces sulfur dioxide, a regulated pollutant.
Sulfides may also be oxidized to less objectionable thiosulfates by air oxidation at 190.degree. F. However, elevated pressures (50-100 psig) are required and the thiosulfates possess considerable chemical and biochemical oxygen demand.
Lastly, small amounts of sulfides can be precipitated with copper (II) or zinc (II) salts. The resulting insoluble sulfides, however, are considered a hazardous waste in that H.sub.2 S will be evolved if the precipitants are exposed to acidic conditions.
It is apparent that new technology is needed in the control of H.sub.2 S production by sulfate reducing bacteria and the treatment of sulfide-laden waters to address the limitations inherent in conventional methods described above.
Thiobacillus denitrificans is a strict autotroph and facultative anaerobe first described in detail by Baalsrud and Baalsrud (Archiv. Mikrobiol. 20, 34 (1954)). Under anaerobic conditions, nitrate may be used as a terminal electron acceptor with reduction to elemental nitrogen. Thiosulfate, elemental sulfur and sulfide may be used as energy sources with oxidation to sulfate; however, sulfide is an inhibitory substrate. It has been demonstrated that T. denitrificans may be readily cultured aerobically or anaerobically in batch or continuous reactors on H.sub.2 S (g) under sulfide-limiting conditions. Complete removal of H.sub.2 S from feed gases was observed with complete oxidation of H.sub.2 S to sulfate which accumulated in the culture media. Stable reactor operation was achieved in batch cultures and continuous cultures at reactor loadings as high as 4-5 mmoles H.sub.2 S oxidized/hr-g biomass. Maximum H.sub.2 S loading of the biomass was estimated at 5.4-7.7 mmoles H.sub.2 S/hr-g biomass under anaerobic conditions and 15.1-20.9 mmoles H.sub.2 S/hr-g biomass under aerobic conditions. Recovery from upset conditions has been demonstrated and heterotrophic contamination of T. denitrificans cultures has been shown to have no effect on H.sub.2 S oxidation.